DESCRIPTION (from applicant's abstract): The auditory steady state response (aSSR) is an electrophysiologic measure of hearing that is gaining increased clinical use as an objective measure of audiologic threshold. Recent reports also suggest dysrythmias in synchronizing activity related to aSSRs may be a common mechanism in chronic pain, Parkinson's disease, and tinnitus. However, the generators of this potential are not fully understood. A more thorough understanding of the neuroanatomy and dynamics of the SSRs will improve their diagnostic value and may lead to new treatment methods for these conditions. The goal of this study is to map the areas and sequence of activation in the brain generating aSSR. Specifically, we will: 1. Use positron emission tomography (PET) to determine the location and distribution of neural structures involved in generating the aSSR. 2. Construct dynamic maps of the location and temporal sequence of activation in the neural network generating the aSSR by combining PET and EEG source localization methods. The electrical activity of the brain during the aSSR will be recorded from the scalp in eight subjects using a 64 channel computerized EEG system. MRI from each subject will provide a detailed anatomic image, clearly segmenting skin, skull, CSF, white matter, and gray matter. The MRI image will be used to construct realistic head models for performing EEG source localization and will serve as a common anatomical space in which to compare PET and EEG data. PET data will be collected using the same stimulus used to elicit the aSSR in the EEG recordings. PET data will also be collected for a quiet resting condition. Comparing the PET data for the aSSR and baseline conditions for significant activations will allow the determination of the neural structures involved in generating the aSSR. EEG source localization constrained by the PET data will then be used to create dynamic maps of the aSSR. These dynamic maps will show the contribution of each structure and the sequence of activation involved in generating the aSSR.